Calibration of a spindle mounted torque transducer normally involves the steps of applying a known torque to the spindle and adjusting the measuring circuitry and/or tranducer to produce an output reading corresponding to the known torque. Obviously, the accuracy of this method of calibration will depend, in large part, on the degree to which the applied torque can be accurately determined and controlled. Up to the present, the easiest way to apply accurately a known torque has been by means of a bench testing unit remote from the machine tool in which the spindle is mounted. Obviously, the machine tool must be at least partially dismantled to permit the spindle to be removed and placed in the bench testing unit for calibration. Following calibration, the spindles must then be reinstalled in the machine tool. The problems associated with this procedure are twofold. First, there is the likelihood of transducer damage after calibration to severe that the transducer is incapable of functioning at all. Although such damage might be detected before the spindle is reinstalled and the machine reassembled, the possibility exists that it would not, thereby necessitating a second dismantling of the machine tool, replacement of the damaged transducer and reassembly of the spindle in the machine tool. Second, there is the potentially more serious problem that, at some point between completion of the calibration on the bench testing unit and reassembly of the machine, the spindle will be bumped or jarred with sufficient severity that the calibration will be disturbed thereby causing inaccurate readings. Such an inaccuracy is not likely to be noticed even after the machine tool is put back into operation. Should the miscalibration resulting during the reassembly process be detected at a later time the machine tool will still have to be dismantled so the transducer can be recalibrated and reinstalled.
Hence, calibration of torque measuring transducers, essential to the optimum performance of machine tools, has been a time consuming and costly procedure. The importance of accurate calibration, however, has required that the process be carried out in the manner described above, despite the time and expense involved.
Any calibration technique which provided the necessary accuracy without requiring removal of the machine spindles would certainly provide a substantial cost advantage. A particularly difficult obstacle to overcome in the development of a practical "on machine" technique for calibrating spindle transducers is the need to accomodate different patterns of spindles within identical multiple spindle machine tools. For example, it is quite common for a manufacturing facility to have several identical multiple spindle machine tools for machining different articles of manufacture, wherein the spindles within each such machine tool are arranged in a unique pattern according to the requirements of the articles being machined thereby. An "on machine" calibration assembly suitable for one arrangement of spindles would not necessarily operate in an identical type of machine tool having a different arrangement of spindles. A more difficult obstacle to overcome occurs when different size and different models of multiple spindle machine tools are used within the same manufacturing facility, since the different work holders within such machines virtually rule out the use of such holders as a means for supporting a practical calibration assembly suitable for each one of the different type machine tools.
One example of an "on line" calibration device for a spindle mounted cutting tool is disclosed in U.S. Pat. No. 3,827,293. The apparatus disclosed therein is mounted on the machine bed and would therefor not be suitable for different types of machine tools. This patent discloses the use of a hydraulic cylinder and a piston which exerts a linear force on the cutting tool spindle and measures spindle deflection. Accordingly, a transducer calibrating apparatus similarly mounted on the work support surface of a multiple spindle torque unit would be useful only for one spindle at a time requiring a complicated dismounting and relocating procedure for each of the remaining spindles in the machine tool.
Other apparatus and methods for checking or calibrating force measuring instruments are disclosed in U.S. Pat. Nos. 3,750,082 to Provenzano et al.; 3,797,305 to Haskell and 4,090,393 to Kharitonov et al. However, none of these discloses a means for applying a known torque, as measured by a master transducer, to calibrate a transducer on a spindle. Moreover, none of these references discloses or suggests an apparatus or method which allows calibration of a transducer on a spindle within the machine where the spindle is located.